mlock() - Unix, Linux System Call

NAME

SYNOPSIS

#include <sys/mman.h>

int mlock(const void *addr, size_t len);

int munlock(const void *addr, size_t len);

int mlockall(int flags);

int munlockall(void);

DESCRIPTION

mlock() and
mlockall() respectively lock part or all of the calling processs virtual address
space into RAM, preventing that memory from being paged to the
swap area.
munlock() and
munlockall() perform the converse operation,
respectively unlocking part or all of the calling processs virtual
address space, so that pages in the specified virtual address range may
once more to be swapped out if required by the kernel memory manager.
Memory locking and unlocking are performed in units of whole pages.

mlock() and munlock()

mlock() locks pages in the address range starting at
addr and continuing for
len bytes.
All pages that contain a part of the specified address range are
guaranteed to be resident in RAM when the call returns successfully;
the pages are guaranteed to stay in RAM until later unlocked.

munlock() unlocks pages in the address range starting at
addr and continuing for
len bytes.
After this call, all pages that contain a part of the specified
memory range can be moved to external swap space again by the kernel.

mlockall() and munlockall()

mlockall() locks all pages mapped into the address space of the
calling process. This includes the pages of the code, data and stack
segment, as well as shared libraries, user space kernel data, shared
memory, and memory-mapped files. All mapped pages are guaranteed
to be resident in RAM when the call returns successfully;
the pages are guaranteed to stay in RAM until later unlocked.

The
flags argument is constructed as the bitwise OR of one or more of the
following constants:

Tag

Description

MCL_CURRENT

Lock all pages which are currently mapped into the address space of
the process.

MCL_FUTURE

Lock all pages which will become mapped into the address space of the
process in the future. These could be for instance new pages required
by a growing heap and stack as well as new memory mapped files or
shared memory regions.

If
MCL_FUTURE has been specified, then a later system call (e.g.,
mmap(2),
sbrk(2),
malloc(3)),
may fail if it would cause the number of locked bytes to exceed
the permitted maximum (see below).
In the same circumstances, stack growth may likewise fail:
the kernel will deny stack expansion and deliver a
SIGSEGV signal to the process.

munlockall() unlocks all pages mapped into the address space of the
calling process.

NOTES

Memory locking has two main applications: real-time algorithms and
high-security data processing. Real-time applications require
deterministic timing, and, like scheduling, paging is one major cause
of unexpected program execution delays. Real-time applications will
usually also switch to a real-time scheduler with
sched_setscheduler(2).
Cryptographic security software often handles critical bytes like
passwords or secret keys as data structures. As a result of paging,
these secrets could be transferred onto a persistent swap store medium,
where they might be accessible to the enemy long after the security
software has erased the secrets in RAM and terminated.
(But be aware that the suspend mode on laptops and some desktop
computers will save a copy of the systems RAM to disk, regardless
of memory locks.)

Real-time processes that are using
mlockall() to prevent delays on page faults should reserve enough
locked stack pages before entering the time-critical section,
so that no page fault can be caused by function calls.
This can be achieved by calling a function that allocates a
sufficiently large automatic variable (an array) and writes to the
memory occupied by this array in order to touch these stack pages.
This way, enough pages will be mapped for the stack and can be
locked into RAM. The dummy writes ensure that not even copy-on-write
page faults can occur in the critical section.

Memory locks are not inherited by a child created via
fork(2)
and are automatically removed (unlocked) during an
execve(2)
or when the process terminates.

The memory lock on an address range is automatically removed
if the address range is unmapped via
munmap(2).

Memory locks do not stack, i.e., pages which have been locked several times
by calls to
mlock() or
mlockall() will be unlocked by a single call to
munlock() for the corresponding range or by
munlockall(). Pages which are mapped to several locations or by several processes stay
locked into RAM as long as they are locked at least at one location or by
at least one process.

LINUX NOTES

Under Linux,
mlock() and
munlock() automatically round
addr down to the nearest page boundary.
However, POSIX.1-2001 allows an implementation to require that
addr is page aligned, so portable applications should ensure this.

Limits and permissions

In Linux 2.6.8 and earlier,
a process must be privileged
(CAP_IPC_LOCK) in order to lock memory and the
RLIMIT_MEMLOCK soft resource limit defines a limit on how much memory the process may lock.

Since Linux 2.6.9, no limits are placed on the amount of memory
that a privileged process can lock and the
RLIMIT_MEMLOCK soft resource limit instead defines a limit on how much memory an
unprivileged process may lock.

RETURN VALUE

On success these system calls return 0.
On error, -1 is returned,
errno is set appropriately, and no changes are made to any locks in the
address space of the process.

ERRORS

Tag

Description

ENOMEM

(Linux 2.6.9 and later) the caller had a non-zero
RLIMIT_MEMLOCK soft resource limit, but tried to lock more memory than the limit
permitted.
This limit is not enforced if the process is privileged
(CAP_IPC_LOCK).

ENOMEM

(Linux 2.4 and earlier) the calling process tried to lock more than
half of RAM.

EPERM

(Linux 2.6.9 and later) the caller was not privileged
(CAP_IPC_LOCK) and its
RLIMIT_MEMLOCK soft resource limit was 0.

EPERM

(Linux 2.6.8 and earlier)
The calling process has insufficient privilege to call
munlockall(). Under Linux the
CAP_IPC_LOCK capability is required.

For
mlock() and
munlock():

EINVAL

len was negative.

EINVAL

(Not on Linux)
addr was not a multiple of the page size.

ENOMEM

Some of the specified address range does not correspond to mapped
pages in the address space of the process.

For
mlockall():

EINVAL

Unknown flags were specified.

For
munlockall():

EPERM

(Linux 2.6.8 and earlier) The caller was not privileged
(CAP_IPC_LOCK).

BUGS

In the 2.4 series Linux kernels up to and including 2.4.17,
a bug caused the
mlockall() MCL_FUTURE flag to be inherited across a
fork(2).
This was rectified in kernel 2.4.18.

Since kernel 2.6.9, if a privileged process calls
mlockall(MCL_FUTURE) and later drops privileges (loses the
CAP_IPC_LOCK capability by, for example,
setting its effective UID to a non-zero value),
then subsequent memory allocations (e.g.,
mmap(2),
brk(2))
will fail if the
RLIMIT_MEMLOCK resource limit is encountered.

AVAILABILITY

On POSIX systems on which
mlock() and
munlock() are available,
_POSIX_MEMLOCK_RANGE is defined in <unistd.h> and the number of bytes in a page
can be determined from the constant
PAGESIZE (if defined) in <limits.h> or by calling
sysconf(_SC_PAGESIZE).

On POSIX systems on which
mlockall() and
munlockall() are available,
_POSIX_MEMLOCK is defined in <unistd.h> to a value greater than 0. (See also
sysconf(3).)